The present invention relates generally to power conversion systems wherein the torque of an AC motor load is controlled through the control of the motor current and flux and more particularly to a method and apparatus which limits the flux angle command signal, over a predetermined operating range, as a function of motor current.
In the past, DC (direct current) motors have been used for operation over a wide speed range as desired. More recently, however, AC motors have been finding greater application in variable speed and drive applications. This is due in a large measure to the inherent ruggedness of an AC induction motor, for example, which exhibits a reduced maintenance problem due to the lack of brushes which makes AC motors particularly desirable for certain applications. Notwithstanding this desirable feature of an AC induction motor, certain problems associated with its use exist particularly when the motor is supplied by power from a variable frequency inverter (DC to AC) which in turn is fed DC power from a converter (AC to DC).
Where, for example, the converter/inverter circuits are implemented by means of phase controlled thyristors connected in bridge circuit configurations, one must consider the commutation delay existing while the current transfer is affected from a previously conducting thyristor to a newly fired thyristor. Normally this overlap is from 10 to 30 electrical degrees. In supplying an AC motor, such as an induction motor at high motor speed, it may take as long as 120 electrical degrees between the firing of a new thyristor and the transfer of the current to the motor phase to which the thyristor is connected. As such, the use of an open loop system in which thyristors are fired as a result of a pre-established schedule is not particularly applicable to the desired type of operation and instead a closed loop system in which thyristors, for example, are fired as a function of the existing angle .theta. between the air gap flux and the motor current, commonly referred to as the air gap power factor, represents a more desirable mode of operation.
In the above cross-referenced related U.S. Pat. No. 4,230,979, which is specifically incorporated herein by reference, there is disclosed a controlled current inverter system for supplying an AC load such as an induction motor with an AC current of variable magnitude and variable frequency. The system described therein employs a variable DC power source which is connected to a variable frequency DC to AC inverter, preferably by way of a DC link including an inductor. Means are included to develop signals representing the instantaneous electrical torque of the AC motor and the instantaneous air gap power factor. The electrical torque signal and the angle signal are then utilized to control the DC current in the link and the firing angle of the inverter with respect to the motor flux such that the angle .theta. is controlled and maintained substantially constant over its prescribed operating range.